This invention relates generally to cable assemblies, and more particularly to flexible cable assemblies of the type used in automobiles for transmitting rotary or linear motion along a predetermined path. In a particular aspect, the present invention relates to abrasion-resistant fluorocarbon polymer composites, such as polytetrafluoroethylene (xe2x80x9cPTFExe2x80x9d) composites, having an unexpectedly high frictional efficiency under high load conditions and after long cycle times. The present invention relates to abrasion-resistant, anti-friction tubing formed from such composites, and to uses of such tubing as a liner for flexible, motion transmitting cable assemblies.
Motion and/or power transmitting cable assemblies are used in a large number of important commercial applications. Perhaps the most common use of such devices occurs in automotive, marine and aircraft installations. Although such cable assemblies are generally hidden from the view of the user, they nevertheless play an important role in many of these well-known modes of transportation. For example, many automobile accessories, such as heaters, air conditioners and side-view mirrors, are dependent upon such assemblies for convenient and reliable operation. Motion transmitting cable assemblies are also frequently indispensable components in the mechanisms used to control critical aspects of vehicle operation. For example, throttle and clutch cables are frequently used to control the speed and power of a vehicle, respectively. It will be appreciated, therefore, that reliable operation of such devices over long periods of use is critical to the safety of present day automobiles.
Furthermore, such assemblies are often exposed to high temperature environments and must be capable of transmitting the required actuating force over relatively long and serpentine paths, with minimal frictional drag; excessive frictional drag may cause extremely dangerous conditions, such as a non-responsive throttle control. In many cases the required actuating force is relatively high, as is the case in connection with clutch and transmission cable assemblies. Accordingly, the provision of cable assemblies which satisfy the above-noted objectives has long been a need in the automobile industry.
Generally, motion transmitting cable systems in common use today comprise a conduit and a motion transmitting core element movably disposed in the conduit. The conduit typically has fittings at each end thereof for attaching the cable assembly to a support structure. In one type of assembly, commonly referred to as a push-pull cable assembly, the cable core is both pushed and pulled to effect remote control of some servient mechanism, apparatus or device. When push-pull cable assemblies are operated in the push mode, the cable core is placed under a compressive load and a substantial lateral load is transmitted to the wall of the associated sheath or conduit. As a result, the side walls of the cable conduit or sheath are frequently subject to intermittent and potentially severe loading, depending upon the mode of operation. Another type of cable assembly is commonly referred to as a xe2x80x9cpull-pullxe2x80x9d cable assembly. In such assemblies, the core element is substantially always operated in tension, never in compression. While such assemblies do produce wear of the cable conduit and its liner, the wear is generally not as severe as with the push-pull type assemblies. In rotary type assemblies, the cable core is rotated in predetermined relation to an operating parameter, such as the speed of a motor vehicle. In such configurations, the conduit is also subject to abrasion as a result of contact with the rotating core.
Fluorocarbon polymers, such as PTFE resins, are well known in the art and have heretofore been utilized in extruded tubular products. Although PTFE resins in their pure form exhibit excellent frictional efficiencies, they generally have unacceptably low abrasion resistance, that is, they wear too rapidly.
The wear resistance of PTFE extruded tubular products has traditionally been enhanced by the inclusion of inert, inorganic fillers such as glass fibers, carbon, asbestos fibers, mica, metals and metal oxides. See, for example, U.S. Pat. No. 3,409,584. While a measure of improvement in wear resistance has thus been achieved, PTFE composites comprising inorganic fillers nevertheless have several disadvantages. For example, such composites generally exhibit rapid deterioration in frictional efficiency after relatively short periods of use. Moreover, the use of such composites as liners for externally lubricated push pull cable assemblies is not generally recommended because the inorganic fillers have been found to separate from the composite and form an abrasive slurry with the lubricant. This abrasive slurry not only decreases frictional efficiency, but it can also cause catastrophic and rapid failure of the liner. As a practical result, therefore, it has previously not been possible to successfully use inorganically filled PTFE composites in lubricated push-pull cable assemblies.
U.S. Pat. No. 4,451,616, issued to Kawachi et al., discloses a process for the preparation of a composite comprising PTFE and an organic filler. Kawachi et al. teach that the both organic and inorganic fillers can be used. More specifically, the filler of Kawachi can be selected from the group consisting of polyimide resins, polyamide-imide resins, polyamide resins and carbon fiber powders. The Kawachi process involves coagulation of PTFE and one of the above mentioned fillers from an aqueous dispersion of these two components. The weight proportion of PTFE and the filler in their aqueous dispersion is disclosed as being from 100:5 to 100:80. Although the patent discloses that the abrasion resistance of PTFE can be enhanced by the incorporation of the above mentioned fillers, there is no indication that any one of those fillers is preferred over another, or that a particular concentration of filler in the composite is preferred.
U.S. Pat. No. 3,391,221, issued to Gore et al., discloses fluorocarbon polymer molding compositions containing from about 10 to about 55 volume percent of what are called xe2x80x9cpermanent lubricant modifiersxe2x80x9d selected from the class consisting of (a) nonvolatile liquids which remain thermally stable and liquid at the sintering temperatures of the fluorocarbon polymer, and have low vapor pressures at those temperatures and (b) materials which are liquid during the forming of the fluorocarbon polymer article and are transformed into a solid in the final shaped article. One important function of the lubricant modifiers of Gore is to act as a lubricating agent during shaping of the polymer. A variety of materials are disclosed as lubricant modifiers, including: aromatic polyamides formed by the reaction of aromatic dicarboxylic acids such as terephthalic acid with aromatic amines such as phenyl diamine or biphenyl diamine; the aromatic polyimides formed by the reaction of such acid dianhydrides as pyromellitic dianhydride with the stated aromatic diamine; the polyamide, polyimide copolymers from the above named components; aromatic polyesters formed from the aromatic dicarboxylic acid and aromatic diols; polybenzimidazoles formed from the aromatic tetracarboxylic acids such as pyromellitic acid and aromatic tetramines; aromatic polyethers; and Novolac epoxy, resins. The only guidance that the patent provides with respect to the selection of modifiers for the enhancement of frictional efficiency is that phenyl silicone lubricants are said to provide high lubricity under high unit loads, and that polymerizable monomers and prepolymers that are polymerized in situ provide molded articles that have a low coefficient of friction. The patent provides no indication that any particular concentration of filler is preferred over another.
U.S. Pat. No. 3,356,759, issued to Gerow, discloses compositions of aromatic polypyromellitimides and a polyfluorocarbon resin. Although this patent broadly refers to the presence of from about 10 to about 90% by weight of fluorocarbon resin in the composite, it expressly teaches that the composite preferably have no more than 50% by weight of the fluorocarbon resin. Accordingly, the Gerow reference teaches composites in which the polyfluorocarbon components preferably constitute a minor proportion of the composite.
Composites comprising a mixture of PTFE and polyarylene sulfide have heretofore been used in fabricating flexible liner or tubing for push-pull cable assemblies. For example, U.S. Pat. No. 4,362,069, issued to Giatras and assigned to the assignee of the present invention, describes a fluorocarbon composite fabricated from a mixture of PTFE resin and a polymer of arylene sulfide. The composite described in this patent, which has exceptional anti-friction, anti-abrasion characteristics, has long been considered one of the preferred products for use in push-pull cable liners. However, despite the success of such products, there is still a need for a liner material which exhibits exceptional anti-friction, anti-abrasion characteristics for a high number of cycles under high load conditions.
One aspect of the present invention provides low cost cable assemblies having reliable and relatively low friction cable operation over long periods of high load use. Applicants have discovered that this and other desirable characteristics are produced by cable assemblies comprising an elongated core for transmitting force or torque along a predetermined path and guide means comprising a liner according to the composition aspects of the present invention.
The abrasion resistant, high efficiency compositions of the present invention comprise a major proportion by weight of fluorocarbon polymer resin, and less than about 10% by weight of polyimide resin filler, preferably having an average particle size of about 20 micron or less. The polyimide resin is preferably a thermoplastic polyimide. Applicants have discovered that unexpectedly superior results are achievable when such a filler is utilized. More particularly, applicants have found that the use of thermoset polyimide resin does not produce performance properties in terms of high load frictional efficiencies that are obtainable in accordance with certain preferred embodiments of the present invention. Furthermore, applicants have discovered the abrasion resistance and frictional efficiency of the present composites is not achievable over long cycle times and with high loads unless the amount of the polyimide included in the composite is as required herein.
According to preferred embodiments of the apparatus aspects of the present invention, the guide means includes a bearing surface comprising a polymer composite of the present invention for resisting abrasion of the guide means as said core moves along the predetermined path. Applicants have found that cable assemblies employing such liners provide results which are not only unexpected in view of the prior art, but which also differ in kind from the results produced by cable assemblies using other liners. According to a preferred aspect of the invention, the polymeric composite liners used in the present cable assemblies have an abrasion resistance of at least about 750,000 cycles of the ambient high load S-test, and even more preferably exhibit a frictional efficiency of at least about 90% over 1,000,000 cycles of the ambient high load S-test.